The long-distance migratory flights of birds are predominantly fueled by the oxidation of fatty acids, which are sourced primarily from extracellular adipose stores. These fatty acids have to be transported, via the circulatory system, to the mitochondria of the active muscles. An important facilitator of fatty acid transport within the cytoplasm of muscle cells is fatty acid binding protein (FABP), which serves as an intracellular carrier of long-chain fatty acids. In mammals, the muscular FABP content is related to the fatty acid oxidation capacity of the tissue. The aim of this study was to measure FABP in samples taken from the cardiac, pectoralis, and semimembranosus muscles of a long-distance avian migrant, the barnacle goose (Branta leucopsis), at various stages of development. Western blot analysis identified a single goose muscle protein of 15 kDa that was able to bind fatty acids and showed a 66% cross-reactivity with antibodies against human heart-type FABP. Captive goslings showed no significant changes in FABP content of either the heart (62.6 +/- 10.6 microgram/g wet wt) or the semimembranosus muscle (8.4 +/- 1.9 microgram/g wet wt) during development. However, in both peripheral and deep sites within the pectoralis muscle, FABP content of samples taken from captive goslings were approximately 10-fold higher throughout development and reached values of 30-40 microgram/g wet wt in fledging goslings at 7 wk of age. A further twofold higher value was seen in wild but not in captive goslings immediately before migration (12 wk of age). Similarly, FABP content was significantly higher in pectoralis samples taken from wild adults (94.3 +/- 3.6 microgram/g wet wt) compared with those from captive adults (60.5 +/- 3.6 micro/g wet wt). These results suggest that the experience of flight activity may be of critical importance in achieving maximal expression of FABP in the pectoralis muscles of postfledging and mature geese immediately before migration.